The present invention relates to an ambient light filter of the type which permits the reliable operation of a light detector circuit whose operation may otherwise be adversely affected by ambient light.
In particular, the invention relates to an ambient light filter which can filter out ambient light which might affect the reading of meters such as those used in the electric utility industry. Particularly, the invention relates to a method and apparatus for filtering ambient light from optical sensor systems used to detect the rotation of a shaft or disk of a meter. This filtering prevents any tampering or erroneous recognition of rotation information. Those skilled in the art will recognize that the term "tampering", as used herein is intended to include any undesired light input in any of the applications in which the present invention may be used (whether or not such undesired light input was intended to deteriorate the operation of the light sensitive circuitry).
While the filtering out of ambient light is important in any application in which such light can affect a reading or signal received by a light sensitive circuit, in the electric utility industry wherein such light sensitive circuits are used to detect and record the movement of a disk within a utility meter, i.e., an electricity meter, such ambient light can be used as a means of "tampering" with the reading ability of the light sensitive circuit. Notwithstanding the specific application for the present invention which will be explained herein, the problem presented (and the current solution) have application in any industry wherein light sensitive circuitry may be used in the presence of ambient light. Such industries include, but are not limited to, the utility metering industry, the communications industry, and numerous applications in which light sensitive circuits are used in the presence of ambient light for counting or security applications.
With respect to the preferred embodiment of the invention, as explained in greater detail herein, electric utilities use attachments to electro-mechanical meters to perform monitoring functions such as demand and time-of-use metering or remote meter reading. The electro-mechanical meters often include a rotating disk or shaft which rotates at a rate proportional to power usage. Sensors or other measuring devices are used to monitor the rotation of the disk or shaft. A standard approach to sensing the rotation of the disk involves the use of light emitting diodes and photosensors. Essentially, the diodes emit light which is sensed by the optical sensors when the disk is in a certain position. This provides an efficient means to track rotation of the disk. Rotation information is stored in electronic registers for use by the utility, which may retrieve the data through various means, including optical port coupling, radio frequency ("RF") reading, telephone modems, and electromagnetic coupling.
Several techniques are used to implement optical sensing. One common arrangement, referred to as "through-hole sensing", involves the placement of one or more apertures along an outer periphery of a rotating disk. The disk separates one or more light emitting diodes from one or more photosensors. As the disk rotates and the apertures pass through the path of the optical sensing area, a photosensor is illuminated. This causes a voltage threshold in the sensing circuit to be exceeded. The voltage threshold is set to correspond to the light exposure condition. As the non-apertured portions of the disk pass between the diode(s) and the sensor(s), light transmission is blocked to prevent the voltage threshold from being exceeded. An electronic register in the meter sensing system is used to store information regarding the rotation of the disk.
In another common arrangement, referred to as "reflective sensing", a disk is provided with one or more darkened light absorbing areas instead of one or more apertures. Light is emitted onto the rotor disk. The light is reflected to a detector if the light hits a reflective portion of the disk. Otherwise, the light is absorbed by a darkened area. Power usage is determined by tracking the number of transitions sensed by the detector circuit. A meter reading scheme using such a variation is disclosed in U.S. Pat. No. 4,327,362, issued to Hoss on Apr. 27, 1982.
Other sensing approaches are also used. Shutters or notches may be placed around the periphery of disks or on a meter disk spindle. Emitters and photosensors are then used to detect movement of these shutters. Examples of such systems are shown in U.S. Pat. No. 4,827,123, issued to Gray on May 2, 1989 and in U.S. Pat. No. 5,241,306, issued to Swanson on Aug. 31, 1993.
Although each of these sensing approaches have slightly different characteristics and features, they share a common need to provide reliable and consistent sensing. The rotation information stored and used by the utility will be in error if the sensing of a meter's rotation fails even temporarily. Current systems suffer in that high levels of ambient light can cause temporary failure of a sensor. This temporary failure can result from fluctuations in naturally-occurring light sources or from customer tampering. Dishonest customers have been known to evade meter sensing devices by directing a light source at the meter sensor.
One approach for handling such abuse involves alerting the utility of interference with a sensor. In this way, a utility may monitor and identify individual users to determine specific instances of abuse. Such an approach is disclosed in the commonly-assigned application entitled METER SENSOR LIGHT TAMPER DETECTOR filed herewith on Mar. 20, 1995, and assigned Ser. No. 08/407,143, now abandoned, which is incorporated herein by reference. However, there are circumstances where it is desirable to do more than just detect the fact of tampering. Instead, it is often desirable to prevent any tampering or interference with a meter sensor.
One common attempt to prevent meter failures from ambient light is to modulate the optical emitter at a known frequency and to design the optical detector to respond to the known frequency by installing an electronic filter in the optical detector's output path. Unfortunately, this approach increases the cost and complexity of the sensor design. Further, it does not prevent a knowledgeable customer from circumventing the filter by modulating a light source at the sensor's modulation frequency.
As automatic meter reading systems, i.e., RF systems become more prevalent, automatic means to recognize that a meter has been tampered with also become more important. This is because a utility will no longer be able to rely upon the meter reader (who formerly read the meter and could observe any obvious tampering) to observe that a meter has been tampered with. Such RF systems are sold by the assignee of the present invention as well as by other companies, and they are described in numerous U.S. patents, including U.S. Pat. No. 4,614,945 of Brunius, et al.; U.S. Pat. No. 4,733,169 of Grindahl; U.S. Pat. No. 4,786,903 of Grindahl, et al.; U.S. Pat. No. 4,799,059 of Grindahl, et al.; and U.S. Pat. No. 4,876,700 of Grindahl.
In view of the above, it would be desirable to provide a method and apparatus for preventing ambient light from interfering with a meter disk sensor without increasing the cost or complexity of a sensor.